Undiscovered Country: Of New Horizons and the Kuiper Belt

By David Darling

Neptune’s largest moon, Triton, seen here in an image from the Voyager 2 flyby, is thought to be a captured Kuiper Belt Object. Photo Credit: NASA.

Starting at about the orbit of Neptune and stretching out to roughly 50 astronomical units (1 AU = Earth-Sun distance), or 7.5 billion kilometers from the Sun, is a region called the Kuiper Belt. It’s like a giant version of the more familiar asteroid belt between Mars and Jupiter—20 times as wide and between 20 to 200 times more massive. We need to learn more about it by having spacecraft fly into it and send back close-up images of some Kuiper Belt Objects (KBOs). The first of these long-distance robot explorers—the New Horizons mission—is well on its way. Astronomers have already discovered more than a thousand KBOs and believe there may be upwards of 100,000 with diameters exceeding 100 km (62 miles). The largest known of them is a world that until recently was considered one of the nine planets of the solar system—Pluto. But the realization that Pluto has an orbit, composition (mostly ices), and size (just 2,300 km across) that fit the KBO bill has led to its reclassification as a dwarf planet.

Known objects (represented by the outer ring of green dots) in the Kuiper Belt. Recent discoveries have highlighted that our knowledge of this region is marginal at best. Image Credit: Minor Planet Center

Other big KBOs include Quaoar, over 1,200 km across, and Makemake, with a diameter of about 1,400 km, and there may be undiscovered bodies in the Kuiper Belt that are larger than Pluto. In fact, Pluto is exceeded in size by an object called Eris, which has taken Pluto’s old slot as ninth biggest and most massive world in orbit around the Sun, but Eris is classified not as a KBO but as a Scattered Disk Object. Scientists also believe that Neptune’s largest moon, Triton, which is slightly larger than Pluto, may be a KBO that was captured long ago by the giant planet’s gravity.

Although four spacecraft from Earth—Voyagers 1 and 2, and Pioneers 10 and 11—have already flown farther from the Sun than the outer edge of the Kuiper Belt, none of these was on a path that allowed study of objects in the Belt. The first spacecraft to be specifically targeted at KBOs is New Horizons, which will encounter Pluto and its collection of moons on July 14, 2015. Pluto’s demotion to dwarf planet status occurred while New Horizons was en route. The spacecraft will send back our first clear views of this largest of KBOs, along with its big moon Charon—itself among KBO heavweights—before heading deeper into the Belt.

The hope is that, having made a thorough survey of the Pluto system, New Horizons will be put on course to make a close flyby of at least one other member of the Kuiper Belt, probably in the size range 50 to 100 km (31 to 62 miles) across—and chosen so that its composition is somewhat different than that of Pluto. Whereas Pluto has a reddish tinge to it, scientists will be looking for the next target to be more white or gray in color, indicative of a different substance on the surface.

New Horizons doesn’t have the ability to drastically alter its course after leaving Pluto, so potential targets will need to lie reasonably close to its flight path. Fortunately our knowledge of smaller KBOs and their whereabouts is set to grow dramatically through ground-based searches such as the Pan-STARRS survey project, which began operations in 2010 and will eventually involve an array of four 1.4 gigapixel digital cameras. Over the next few years Pan-STARRS is expected to increase the number of known KBOs 20-fold, enabling candidates to be chosen for New Horizons to explore.

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There are a few errors in this article. First, Pluto is not composed mostly of ices. Its composition is estimated to be 70 percent rock. Eris is not bigger than Pluto. It was initially thought to be so, but in November 2010, a team of astronomers led by Dr. Bruno Sicardy determined it is marginally smaller than Pluto though more massive. Being more massive likely means it is more rock. Nothing about Pluto’s or Eris’s orbits disqualify it from being a planet, at least according to the equally legitimate geophysical planet definition, in which a planet is any non-self-luminous spheroidal body orbiting a star. Pluto and Eris are well beyond the threshold for being rounded by their own gravity, which is why both are small planets. These objects are both Kuiper Belt Objects and planets. The first tells us where they are; the second what they are. They are complex worlds with geology, weather, and differentiation, making them unlike the majority of tiny bodies in the Kuiper Belt.

The definition of planet set in 2006 by the International Astronomical Union (IAU) states that, in the Solar System, a planet is a celestial body which:

1)is in orbit around the Sun,
2)has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3)has “cleared the neighbourhood” around its orbit.

A non-satellite body fulfilling only the first two of these criteria is classified as a “dwarf planet”
(source: Wikipedia)

Pluto occupies a region in the solar system that is shared by possibly hundreds of thousands of other similal bodies. So, it’s right to call Pluto a ‘dwarf planet’

The case was similar with the discovery of the asteroids in the main asteroid belt between Mars and Jupiter. When Ceres was discovered in 1801, everybody thought it was a new planet, because it was also round. Later, hundreds of similar bodies were being discovered, making astronomers to re-classify the objects as asteroids. So the whole case with Pluto is nothing new actually.

The 2010 occultation data put the two at almost exactly the same size — a statistical tie. We’ll need further measurements to be a able to say for certain which is the larger. New Horizons will help with that by pinning down the exact size of Pluto. Yes, the ice composition of Pluto is estimated at 30-50%, so Pluto is probably more rocky than icy. Pluto and Eris are both classed as KBOs and dwarf planets by the IAU.

I wonder what would have been the additional cost for New Horizons to have been equipped with a small lander or probe to be placed on the surface of Pluto as the spacecraft made its flyby. I’ll bet the value of the information returned from the surface of Pluto would have made the investment worthwhile.

Yes, that would be the case indeed. But I suspect that NASA wanted to launch the probe as fast as possible, because Pluto had already passed perhelion in its orbit, and by each passing year the atmosphere was beginning to freeze over, and it wouldn’t be long unti it was all frozen solid on Pluto’s surface and any observations wouldn’t have any meaning by then. An additional rover for the mission would probably have delayed the launch more.

The planet definition set in 2006 was highly controversial and was made by only four percent of the IAU. It was opposed by hundreds of professional astronomers in a formal petition led by New Horizons Principal Investigator Dr. Alan Stern. You can find the petition here: http://www.ipetitions.com/petition/planetprotest/ . That third IAU criterion is rejected by many astronomers. Pluto does not share its orbit with thousands of other similar bodies. Those bodies are tiny iceballs while Pluto is a complex world in hydrostatic equilibrium, with geology, weather, and layering. The term “dwarf planet” was coined by Dr. Stern to designate a third class of planets in addition to terrestrials and jovians, not to refer to non-planets. You example of Ceres is actually based on an error. Ceres should never have been demoted; however, 19th century astronomers’ telescopes could not resolve it into a disk, so they did not know it is spherical. Today, we do know it is in hydrostatic equilibrium and therefore a small planet. The IAU can classify objects however it wants; that does not mean their way becomes gospel truth. To adherents of a geophysical planet definition, dwarf planets are planets. Pluto and Eris are both KBOs and small planets (of the dwarf planet subcategory). The first tells us where they are; the second tells us what they are. The two categories are not mutually exclusive.

Karol raises an interesting point regarding a lander attached to the Pluto spacecraft. Obviously too late and, as Jim Oberg points out, probably too late for a trams-Plutonian target. However, mission planners would be wise to include landers as “standard equipment” on all future planetary spacecraft.

Tom, I don’t think that any member of the space community who has seen the astonishing image of the landscape of Saturn’s moon Titan (so amazingly clear that one feels as though they could reach out and touch a rock) as taken by the Huygens lander from the Cassini spacecraft would disagree with you. However Tom, I’ll bet you a top-shelf bottle of Scotland’s finest that more people have seen the Kardassian baby picture than have seen this historic image of an alien world transmitted across a billion kilometers of interplanetary space.

Well Laurel, I don’t have exact numbers, but Dr. Neil DeGrasse Tyson once said that adding up all of the individuals who are “space advocates” like members of The Planetary Society and other such groups, together with individuals who are actively working in the space industry, there are only about 300,000 of us. He further points out that even the Hannah Montana Fan Club has over a million members. Dr. Tyson wanted to make it very clear that if we really want a viable program of space exploration, what we lack in numbers, we have to make up for in effort. Oh, and I have a framed image of the landscape of Titan, and the picture of North West I saw on the news was of one adorable little baby. 🙂

I am enjoying all these comments, as most of the information here answers questions I’ve wondered about myself. I do have a couple though that I can’t find satisfactory answers to that hopefully someone here will be able to help me with. First, regarding the Titan lander images, does anybody happen to know why the field of view of the lander camera was so narrow (unlike Mars/Moon landers that have wide panoramas and mounts that rotate to give a 360 degree perspective of the landscape)? I should think that such a sophisticated lander with such a rare opportunity as this would have been able to secure pictures of a much broader surface area than what we got (although of course, what we got was certainly better than nothing, don’t get me wrong). Second, although we are still two years away from the Pluto flyby, looking at the trajectory maps for New Horizons’ current position, shouldn’t they be close enough now to be able to take occasional photos of Pluto (since the camera lens is telescopic)so that as they continue to get closer and the photos get better they can start being studied in advance, to search for signs of any more possible moons so that the spacecraft path can be adjusted accordingly in time if more are found? It would seem like the most practical thing to do instead of waiting until we get there and then having to play “Asteroids” with whatever else might be there at the spur of the moment (since a “moment” in terms of radio signals getting back to us from Pluto and then back to Pluto from Earth takes over 3 hours each way…not much time to do any fancy maneuvering if need be!) Thanks guys for any input you can give on these questions!

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